The rectus sheath is a strong, fibrous compartment located in the anterior abdominal wall. Formed by the broad, flat tendons (aponeuroses) of the lateral abdominal muscles, it creates a durable layer of connective tissue. This protective enclosure is important for core stability, maintaining the integrity of the abdomen, and providing a fixed anchor point for muscle contraction. The sheath allows for coordinated movement and protects deeper structures.
Defining the Structure and Location
The rectus sheath is situated along the midline of the body, extending vertically from the lower rib cage down to the pubic crest in the pelvis. It is formed by the aponeuroses of the external oblique, internal oblique, and transversus abdominis muscles. These three lateral muscles transition into their aponeuroses as they approach the center of the abdomen, where they interweave to form the sheath.
The sheath’s boundaries start superiorly near the xiphoid process and the costal cartilages of the fifth to seventh ribs. Inferiorly, the structure extends to fuse with the pubic crest and the pubic symphysis. The point where the aponeuroses from both sides meet and fuse at the midline is known as the Linea Alba. This central fusion point provides a strong attachment for the abdominal wall musculature.
Layers and Composition of the Rectus Sheath
The composition of the rectus sheath varies based on its location relative to the arcuate line. This structural change is a key feature of the anatomy. The arcuate line is a curved, horizontal demarcation found on the posterior wall of the sheath, typically located about one-third of the distance from the navel to the pubic bone.
Above the arcuate line, the aponeurosis of the internal oblique muscle splits into two layers to surround the muscle it encloses. The anterior wall is composed of the external oblique aponeurosis and the anterior half of the internal oblique aponeurosis. The posterior wall is formed by the posterior half of the internal oblique aponeurosis and the entire aponeurosis of the transversus abdominis muscle.
This arrangement fully encloses the muscle with aponeurotic tissue in the upper two-thirds of the abdomen. Below the arcuate line, the structure changes, resulting in a significant decrease in posterior support. The aponeuroses of all three lateral abdominal muscles—the external oblique, internal oblique, and transversus abdominis—all pass entirely to the front of the muscle.
This consolidated structure below the arcuate line creates a thick anterior wall. Consequently, the posterior wall of the rectus sheath disappears, leaving only the transversalis fascia separating the muscle from the abdominal cavity. This transition point, where the fibrous posterior support ends, is important for understanding the anatomy of the lower abdomen.
Structures Contained Within the Sheath
The rectus sheath primarily houses the Rectus Abdominis muscle, often called the “six-pack” muscle. This muscle is responsible for flexing the trunk and assisting in maintaining intra-abdominal pressure. The smaller Pyramidalis muscle is also located in the sheath near the pubic bone, though it is absent in about 20% of the population.
The sheath provides passage for the neurovascular supply to the anterior abdominal wall. Blood flow is supplied by the superior and inferior epigastric arteries and veins, which run vertically along the posterior aspect of the muscle. The inferior epigastric vessels pass across the arcuate line to enter the sheath from below, while the superior epigastric vessels descend into it from above.
The terminal ends of the thoracoabdominal nerves, which are branches of the lower thoracic spinal nerves (T7 through T12), also travel within the sheath. These nerves provide motor control to the muscle and sensory innervation to the overlying skin.
Clinical Significance and Common Issues
The rectus sheath’s structural variations have direct implications for medical conditions and surgical procedures. The strength of the sheath is important for preventing the outward protrusion of abdominal contents, which can lead to hernias. The relative weakness below the arcuate line, caused by the lack of a strong posterior aponeurotic layer, makes this area susceptible to certain issues.
A common issue is a Rectus Sheath Hematoma, which is bleeding into the compartment, often caused by the rupture of an epigastric artery or a muscle tear. Since the epigastric vessels run along the posterior sheath, bleeding is often contained above the arcuate line by the intact posterior wall. Below the arcuate line, the absence of this strong posterior containment allows blood to spread more easily.
The sheath is an important surgical landmark, especially in procedures involving abdominal wall repair or access to the abdomen. Surgeons use the layers of the sheath to guide incisions and repairs, ensuring the layers are closed precisely to restore the wall’s integrity. Understanding the location of the arcuate line and the course of the epigastric vessels helps minimize complications during surgery.